1. Field of the Invention
The present invention relates to ferroelectric thin film device that functions as a piezoelectric device, a nonvolatile ferroelectric memory device, a pyroelectric device, or the like, and more particularly to a technique for controlling the orientation of a ferroelectric thin film, and to a technique for improving the bottom electrode of this ferroelectric thin film device.
2. Description of the Related Art
Crystalline materials consisting of compound oxides that exhibit ferroelectricity, such as lead titanate zirconate, barium titanate, and lithium niobate, have numerous functions, including spontaneous polarization, a high dielectric constant, an electro-optical effect, a piezoelectric effect, and a pyroelectric effect, and as such are used in the development of a wide range of devices. For instance, the piezoelectric properties of these materials are utilized in capacitors in FRAM (Ferroelectric Random Access Memory), DRAM (Dynamic Random Access Memory), and the like, their pyroelectric properties are utilized in infrared linear array sensors, and their electro-optical effect is utilized in wave-guide type light modulators, so these materials can be used in many different fields. Ferroelectric thin film devices having these various functions are also called functional devices.
It is often the case with a ferroelectric thin film device such as this that the characteristics vary with the crystal orientation of the ferroelectric thin film. For example, most lead titanate-based ferroelectrics, which are a type of ferroelectric having a perovskite type crystal structure, have a tetragonal crystal structure, and have spontaneous polarization in the c axis direction. Accordingly, spontaneous polarization in the direction perpendicular to the substrate can be maximized by orienting the c axis to be perpendicular to the substrate (c axis orientation treatment), allowing the performance of a ferroelectric thin film device in which this characteristic is utilized to be utilized to full advantage.
For this reason, it is important to control the crystal orientation in the formation of a ferroelectric thin film, and particularly a lead titanate-based ferroelectric film having a perovskite type crystal structure. Furthermore, since the electrical characteristics of these ferroelectric thin films vary with the orientation of the crystal plane, an orientation treatment must be performed according to the intended application of the ferroelectric thin film device. For example, a (100) priority orientation is known to be preferable with a nonvolatile ferroelectric memory device that makes use of the polarization characteristics of a ferroelectric thin film, such as a FRAM. With a piezoelectric device that is utilized as an electromechanical transducer (actuator), in the case of DC drive, a (111) priority orientation is known to be preferred because of the piezoelectric constant d31 characteristics in the drive voltage region.
Except in the case of natural orientation, the crystal orientation of a ferroelectric thin film is affected by the crystal orientation of the bottom electrode or substrate that serves as the base in the formation of the ferroelectric thin film. Accordingly, proper selection of the material of the bottom electrode or substrate that serves as the base is absolutely essential to controlling the orientation of a ferroelectric thin film. Generally, a silicon substrate is used as the substrate of a ferroelectric thin film, and a silicon dioxide film is formed in order to ensure good electrical insulation between the bottom electrode and the substrate, so a required characteristic of the bottom electrode is that it have good orientability even when formed on an amorphous film. Platinum electrodes have been used in the past as electrodes that satisfy this requirement. The lattice constant of a platinum electrode is matched to that of lead titanate zirconate, and because platinum is resistant to oxidation, no platinum oxide layer is formed at the interface with the dielectric layer, so the performance of the device tends not to deteriorate.
As to technology related to the bottom electrode, it has been reported in Japanese Patent Laid-Open No. 07-245236 that a structure having an iridium layer or an alloy layer of platinum and iridium as the bottom electrode is favorable in terms of the matching of the lattice constants of the bottom electrode and PZT. Japanese Patent Laid-Open No. 08-335676 deals with an improvement on this technology, reporting that if nuclei of a component element of PZT (titanium) are formed on the bottom electrode in a structure having an iridium layer or an alloy layer of platinum and iridium as the bottom electrode, crystals will grow around the nuclei and good contact with the PZT film can be ensured.
As to technology related to the substrate that serves as a base, it has been reported in Japanese Patent Laid-Open No. 5-281500 that a lithium niobate thin film is formed by sol-gel method on a sapphire (001) plane monocrystalline substrate. According to this technique, the axis of crystallization of a ferroelectric thin film can be uniaxially oriented by utilization of the crystallinity of the substrate.
However, even though it was possible to form a ferroelectric thin film with excellent orientability by optimizing the conditions that affect the orientation of a ferroelectric thin film by means of the bottom electrode (or substrate), such as matching the lattice constants of the bottom electrode (or substrate) and the ferroelectric thin film, as with the above-mentioned prior art, it was difficult to control the orientation of a ferroelectric thin film as desired according to the intended application of the ferroelectric thin film. For instance, if an attempt was made to vary the film formation conditions in the formation of a PZT film by sol-gel method, it was difficult to control the (100) priority orientation, which is favorable for a nonvolatile ferroelectric memory device, and the (111) priority orientation, which is favorable for the DC drive of an electromechanical transducer, as desired.
Also, diligent study by the inventors revealed that when a ferroelectric thin film device is used as an electromechanical transducer, the piezoelectric constant d31 will be higher if the ferroelectric thin film is set to a priority orientation of (111) in a drive frequency band of just a few kHz (low frequency band), and the piezoelectric constant d31 will be higher if the ferroelectric thin film is set to a priority orientation of (100) in a drive frequency band of several dozen kHz (high frequency band). This seems to be because the piezoelectric constant d31 remains more or less constant regardless of the drive frequency if the ferroelectric thin film is set to a priority orientation of (100), whereas the piezoelectric constant d31 decreases in value as the drive frequency goes up if the ferroelectric thin film is set to a priority orientation of (111). It is therefore desirable to be able to control as desired the orientation of a ferroelectric thin film according to the drive frequency of the electromechanical transducer.
Also, the technology disclosed in Japanese Patent Laid-Open No. 08-335676 allows crystals to be grown around nuclei and good contact with a PTZ film ensured by forming nuclei of a component element of PZT (titanium) on the bottom electrode, but if iridium alone was used as the bottom electrode and the PZT film was formed by sol-gel method, then there was a problem in that the bottom electrode took in oxygen and swelled in the course of the baking of the PZT film. Because the bottom electrode became hard and brittle if it took in oxygen, the bottom electrode would break if used as an actuator.
The structure that used to be employed when a ferroelectric thin film device was used as an electromechanical transducer had an adhesive layer (buffer layer) of titanium, chromium, or the like provided between the bottom electrode and the surface where this transducer was installed in order to enhance the adhesion between the electromechanical transducer and this installation surface. The inventors of the present invention, however, discovered that in the course of the manufacture of an electromechanical transducer, the element that makes up the adhesive layer, such as titanium, is diffused as a result of heat treatment and moves into the ferroelectric thin film, which diminishes the piezoelectric characteristics of the electromechanical transducer. The reason for this seems to be that the titanium becomes admixed with the ferroelectric thin film and disrupts the stoichiometric ratio in this film, or produces a layer with a low dielectric constant at the interface between the bottom electrode and the ferroelectric thin film.
A ferroelectric thin film has spontaneous polarization, and because the polarization direction can be inverted by the action of an external electrical field, this characteristic can be utilized to manufacture a nonvolatile memory. When a ferroelectric thin film was applied as a memory device, an alloy of platinum and iridium, iridium alone, or iridium oxide was used in the past as a bottom electrode for applying an electrical field to the ferroelectric thin film in an effort to enhance the characteristics of the ferroelectric thin film and prevent their deterioration over time. Examples of such usage are found in U.S. Pat. No. 5,191,510, Japanese Patent Laid-Open No. 07-245287, and elsewhere.
However, the bottom electrodes composed of an alloy of platinum and iridium, iridium alone, or iridium oxide that were used for memory devices posed problems when used as electromechanical transducers, such as in an ink jet recording head. Specifically, the electromechanical transducer must itself be deformed with an ink jet recording head, but with a bottom electrode containing iridium, there were problems in that the bottom electrode was too hard and the film stress generated in the bottom electrode was too high. Another problem was poor adhesion between the bottom electrode and the installation surface and between the bottom electrode and the ferroelectric thin film.
Furthermore, the problem of markedly increased leakage current was encountered when iridium was used as the bottom electrode, as discussed in the article xe2x80x9cExplanation of the Leakage Mechanism of a PZT Capacitor Deposited on an Ir Electrode (Lecture Summaries from the 59th Convention of the Applied Physics Society, issued Sep. 15, 1998, p. 450). Accordingly, the use of platinum is normally preferred for the bottom electrode when a ferroelectric thin film is used as an actuator for an ink jet recording head.
Platinum is therefore favorable for the bottom electrode of a electromechanical transducer, but there has been a need for a bottom electrode having a structure capable of preventing the admixture of impurities (such as titanium) into the ferroelectric thin film during the electromechanical transducer manufacturing process, and of enhancing the adhesion between the bottom electrode and the installation surface and between the bottom electrode and the ferroelectric thin film.
When an alloy of platinum and iridium was used for the bottom electrode of an electromechanical transducer as disclosed in Japanese Patent Laid-Open No. 07-245236, or when iridium oxide was used as disclosed in Japanese Patent Laid-open No. 07-245237, a problem was encountered in that residual stress was generated in the bottom electrode containing iridium in the course of baking and crystallizing the ferroelectric thin film, and this residual stress diminished the characteristics of the electromechanical transducer. For example, the residual stress generated in a bottom electrode could impart strain to the ferroelectric thin film and undesirably lower the percentage of volumetric change.
In view of this, it is an object of the present invention to provide a method for manufacturing a ferroelectric thin film device with which the crystal orientation of a ferroelectric thin film can be controlled as dictated by the intended application of an electromechanical transducer. It is a further object to provide an ink jet recording head whose ink discharge drive source is an electromechanical transducer obtained by this manufacturing method, as well as a method for manufacturing this head, and an ink jet printer that makes use of the same. Another object of the present invention is to provide a nonvolatile ferroelectric memory device in which a ferroelectric thin film device obtained by this manufacturing method serves as a capacitor, and a method for manufacturing this memory device.
Yet another object of the present invention is to provide a method for manufacturing an electromechanical transducer having very reliable drive characteristics when iridium alone is used as the material for the bottom electrode.
Yet another object of the present invention is to provide an electromechanical transducer with which adhesion with the installation surface can be enhanced without diminishing the piezoelectric characteristics, and an ink jet recording head and an ink jet printer that make use of this transducer. Another object of the present invention is to provide a method for manufacturing an electromechanical transducer having a layer structure with which adhesion with the installation surface can be maintained without diminishing the piezoelectric characteristics.
Still another object of the present invention is to provide an electromechanical transducer having good piezoelectric characteristics as a result of reduced residual stress during baking, an ink jet recording head and printer that make use of this electromechanical transducer, and a method for manufacturing an electromechanical transducer.
With the method of the present invention for manufacturing a ferroelectric thin film device, a bottom electrode film containing at least iridium is formed on a surface preparation layer whose main component is zirconium oxide, and an ultra-thin titanium layer is laminated over this bottom electrode. Next, a crystallized ferroelectric thin film is formed by forming an amorphous layer containing elemental metal and elemental oxygen that constitute a ferroelectric over the titanium layer, and heat treating the amorphous layer. It was confirmed that the orientation of the ferroelectric thin film can be controlled by adjusting the film thickness during the lamination of the titanium layer at this point. For instance, if the thickness of the titanium layer is at least 2 nm and less than 10 nm, the ferroelectric thin film will have a (100) priority orientation, and if this thickness is at least 10 nm and less than 20 nm, the ferroelectric thin film will have a (111) priority orientation.
Therefore, a ferroelectric thin film device that is favorable as a electromechanical transducer used with DC drive or low frequency drive will be obtained by setting the thickness of the titanium layer to at least 10 nm and less than 20 nm, whereas a ferroelectric thin film device that is favorable as a capacitor for a nonvolatile ferroelectric memory device, or an electromechanical transducer that is favorable for high frequency drive will be obtained by setting the thickness of the titanium layer to at least 2 nm and less than 10 nm.
It is preferable for the ferroelectric thin film to be a ferroelectric whose constituent components are at least titanium and lead, and lead titanate zirconate is particularly favorable. It is preferable for the ferroelectric thin film to be formed by sol-gel method. A sol-gel method is preferred in terms of orientation control because the crystallization of the ferroelectric thin film proceeds from the bottom electrode side.
It is also preferable for the bottom electrode to be a single layer of an iridium film or a laminate film having a laminated structure comprising an (iridium layer)/(platinum layer), a (platinum layer)/(iridium layer), or an (iridium layer)/(platinum layer)/(iridium layer), in that order starting at the surface preparation layer.
The ink jet recording head of the present invention comprises an electromechanical transducer obtained by the manufacturing method of the present invention, a pressure chamber whose internal volume is varied by the mechanical displacement of an electromechanical transducer, and discharge outlets that communicate with the pressure chamber and from which ink droplets are discharged. The ink jet printer of the present invention has a printing function comprising the ink jet recording head of the present invention.
In the method of the present invention for manufacturing an ink jet recording head, a surface preparation layer whose main component is zirconium oxide is formed on a silicon substrate surface, either directly or via a diaphragm film, and an electromechanical transducer is formed by the above-mentioned manufacturing method of the present invention over this surface preparation layer. The electromechanical transducer is then separated so as to line up with a position where the mechanical displacement of the electromechanical transducer can be imparted to the pressure chamber.
In the method of the present invention for manufacturing a nonvolatile ferroelectric memory device, there is a step for manufacturing the capacitor of a memory cell by the above-mentioned manufacturing method of the present invention.
The method of the present invention for manufacturing an electromechanical transducer comprises the steps of forming a bottom electrode composed of iridium alone over a surface preparation layer whose main component is zirconium oxide, laminating a titanium layer whose film thickness is at least 15 nm and less than 30 nm over this bottom electrode, and forming a crystallized ferroelectric thin film by forming an amorphous film containing the elemental metal and elemental oxygen that constitute the ferroelectric over said titanium layer and then heat treating this amorphous film.
The oxygen content of the bottom electrode in the course of baking the ferroelectric thin film can be kept to a minimum and an electromechanical transducer with excellent toughness can be provided by adjusting the thickness of the titanium layer laminated over the bottom electrode to within a range of at least 15 nm and no more than 30 nm.
The priority orientation of the ferroelectric thin film can be controlled to the (111) plane or the (110) plane by adjusting the thickness of the titanium layer to within the above range. The step of forming the ferroelectric thin film is preferably a sol-gel process or MOD process.
The electromechanical transducer of the present invention further comprises an adhesive layer formed from an alloy containing an anti-diffusion metal and formed between the bottom electrode and the surface where the transducer is installed, and an anti-diffusion layer formed from an alloy containing the anti-diffusion metal and formed between the bottom electrode and said ferroelectric thin film.
The anti-diffusion metal is selected, for example, from the group consisting of iridium, palladium, rhodium, ruthenium, and osmium. The above-mentioned adhesive layer is, for example, an alloy of the anti-diffusion metal and the metal that constitutes the bottom electrode. The above-mentioned anti-diffusion layer is, for example, an alloy of the anti-diffusion metal and an adhesive metal that is either titanium or chronium. The bottom electrode is made of platinum.
It is preferable for the ferroelectric thin film to be formed in a thickness of at least 1 xcexcm. The baking treatment must be repeated numerous times for this thickness to be achieved, but the diffusion of the titanium or other adhesive metal is prevented by the anti-diffusion layer of the present invention even though the baking treatment is performed numerous times.
The ink jet recording head of the present invention is constituted by an arrangement of the electromechanical transducers of the present invention on the diaphragm film that forms at least one side of a pressure chamber filled with ink. The diaphragm film is, for example, constituted by the lamination of a silicon oxide film with a zirconium oxide film or the like. The ink jet printer of the present invention is a printer furnished with this ink jet recording head as an ink discharge means.
The method of the present invention for manufacturing an electromechanical transducer comprises the steps of forming an adhesive metal layer composed of an adhesive metal over the surface where the transducer is installed, forming a first anti-diffusion metal layer composed of an anti-diffusion metal over the adhesive metal layer, forming the bottom electrode over the anti-diffusion metal layer, forming a second anti-diffusion metal layer composed of the anti-diffusion metal over the bottom electrode, and baking the ferroelectric thin film while this ferroelectric thin film is formed over the second anti-diffusion metal layer, and thereby diffusing the adhesive metal all the way to the second anti-diffusion metal layer and producing an anti-diffusion layer at the location of the second anti-diffusion metal layer, promoting the alloying of the anti-diffusion metal and the bottom electrode, and producing an adhesive layer at the location of the adhesive metal layer and first anti-diffusion metal layer.
Preferably, a metal selected from the group consisting of iridium, palladium, rhodium, ruthenium, and osmium is used as the anti-diffusion metal. Also, either titanium or chronium is used as the adhesive metal.
The electromechanical transducer of the present invention comprises an interlayer formed from a compound selected from the group consisting of zirconium oxide, tantalum oxide, silicon nitride, and aluminum oxide and formed on the surface where the transducer is installed, and a bottom electrode formed over this interlayer. The bottom electrode comprises a first layer composed of an alloy of iridium and a specific metal and provided over the interlayer, and a second layer containing iridium and provided over the first layer.
This structure is formed when the baking is performed at a relatively low temperature of 750xc2x0 C. or lower, for example, and there is little movement of the iridium.
The electromechanical transducer in another embodiment of the present invention comprises an interlayer formed from a compound selected from the group consisting of zirconium oxide, tantalum oxide, silicon nitride, and aluminum oxide and formed on the surface where the transducer is installed, and a bottom electrode formed over the interlayer. The bottom electrode comprises a first layer containing a specific metal and provided over the interlayer, and a second layer containing iridium and provided over the first layer.
This structure is formed when the baking is performed at a relatively high temperature of over 750xc2x0 C., for example, and there is much movement of the iridium.
The above-mentioned second layer is characterized in that the iridium that has diffused from the lower layer side is separated from the iridium present from the outset. An adhesive layer composed of a metal that adheres to both the interlayer and the bottom electrode may also be formed between these layers.
For example, it can be checked that the volumetric ratio in the bottom electrode accounted for by the alloy containing iridium is at least ⅖ and no more than ⅘. The xe2x80x9calloy containing iridiumxe2x80x9d refers to an alloy of iridium and titanium, oxygen, or the like.
The ink jet printer of the present invention is characterized in that the electromechanical transducer of the present invention is provided as an actuator over the diaphragm film that serves as the installation surface. The ink jet printer is also characterized by comprising this ink jet recording head as a printing means.
The method of the present invention for manufacturing an electromechanical transducer comprises the steps of using a compound selected from the group consisting of zirconium oxide, tantalum oxide, silicon nitride, and aluminum oxide to form an interlayer on the surface where this transducer is installed, forming a bottom electrode over this interlayer, forming a ferroelectric thin film precursor over this bottom electrode, and baking.
Here, the step of forming the bottom electrode comprises the steps of using iridium to form a first iridium layer, using a specific metal to form a metal layer over the first iridium layer, and using iridium to form a second iridium layer over the metal layer, the baking step being a step of forming the ferroelectric thin film precursor and then baking it at a temperature of 750xc2x0 C. or lower, thereby diffusing the iridium of the first iridium layer and converting the first iridium layer and the metal layer into an alloy layer in which iridium is alloyed with the metal.
The method for manufacturing an electromechanical transducer in another embodiment of the present invention comprises the steps of using a compound selected from the group consisting of zirconium oxide, tantalum oxide, silicon nitride, and aluminum oxide to form an interlayer on the surface where this transducer is installed, forming a bottom electrode over the interlayer, forming a ferroelectric thin film precursor over the bottom electrode, and baking.
Here, the step of forming the bottom electrode comprises the steps of using iridium to form a first iridium layer, using a specific metal to form a metal layer over the first iridium layer, and using iridium to form a second iridium layer over the metal layer.
The baking step is a step of forming the ferroelectric thin film precursor and then baking it at a temperature higher than 750xc2x0 C., thereby diffusing the iridium of the first iridium layer and moving the iridium of the first iridium layer to the second iridium layer.
Preferably, the ratio of the thickness of the first iridium layer prior to baking to the thickness of the bottom electrode overall is set to be between ⅓ and ⅘. The reason for this is that stress will be moderated more efficiently if the thickness of the iridium layer is within this range.
The method of the present invention for manufacturing an electromechanical transducer may further comprise the step of using a metal that will adhere to the layers above and below to form an adhesive layer between the bottom electrode and the interlayer.
Here, the step of forming a bottom electrode is a step of forming a film such that the following relationship is satisfied:
dT=3.6xc3x97d0+2.4xc3x97d1+0.8xc3x97d2+2.3xc3x97d3
when we let d0 be the thickness of said adhesive layer prior to baking, d1 be the thickness of said first iridium layer, d2 be the thickness of said metal layer, d3 be the thickness of said second iridium layer, and dT be the thickness of said bottom electrode overall after baking. The reason for this is that the thickness of the layer after crystallization varies as indicated by the above relationship as a result of baking after formation in this relationship.